1. Field of the Invention
The present invention relates to an impact-fatigue test machine for steel or cast iron balls which are used as grinding balls.
2. Description of the Prior Art
As a result of an increased scale of ball mills and improved ball mill liners, grinding balls are subjected to greater impact forces. Impact resistance of grinding balls is represented mainly by fracture toughness. The conventional impact tests used to determine impact resistance are Izod tests, Charpy tests, and tensile strength tests. These tests, however, fail to be an adequate means of testing impact resistance due to the difficulties in preparing test pieces from the hard materials of which grinding balls are made and due to the tests' inabilities to simulate the actual impact forces experienced in ball mills. In the case of cast-iron grinding balls, it was impossible to obtain accurate measured values of impact resistance by means of the above mentioned tests. The tests fail to evaluate the difference in actual fracture toughness of a ball based on a slight difference in the material of the balls.
In a ball mill that portion of a ball which impacts another ball is subjected to complex, three-dimensional stresses. The above-mentioned tests fail to provide results that correlate well with the actual fracture toughness or the impact fatigue life of grinding balls in a ball mill.
Recently, various types of impact fatigue test machines have been proposed or developed in an attempt to evaluate the fracture toughness of grinding balls under repeated conditions. For example, a test machine of this kind is disclosed in the Journal of the Iron and Steel Institute, 197, (1961), PP. 40-48 (R.H.T. Dixon). This machine is depicted in FIG. 1 and includes an anvil 101 made of Ni-Hard cast iron, with an upper surface inclined at a 30 degree angle. Each grinding ball to be tested is individually carried by means of a lift conveyor 102 up to an upper chute 103, which introduces the ball into the upper end of a drop-guide member 104.
The ball drops from the upper end of the drop guide member 104 vertically downwards and impacts upon the anvil 101 lying below. Upon such impact, the ball rebounds several times against a rebound plate 104. After the rebounding motion has stopped, the ball rolls along a lower chute to the lower portion of the lift conveyor to repeat the cycle. This system then, subjects a ball to repeated impact cycles. The cycle is stopped when the ball fractures. Foundry Trade Journal, 1973, 5, P. 645, describes the test results obtained by the machine shown in FIG. 1 which was used to test balls made of nodular cast iron, high chrome steel based on, austenite, high chrome steel based on martensite etc. The journal concludes that the effect of anvil 101 on the test results cannot be excluded.
In Symposium, Materials for the Mining Industry, 1974, PP. 189-195, the test results obtained by the above-mentioned machine for balls of white cast iron and other materials are discussed. The results of these tests failed to correlate well with the actual life and toughness of balls used in ball mills.